The standard NTSC color television signal currently used in the United States and variations of that signal in systems derived from it, such as the PAL and SECAM systems, are comprised of color information signal component, phase and amplitude demodulated on a color subcarrier to represent hue and saturation, respectively; a brightness signal component; a burst signal component, synchronized with the color information subcarrier; and synchronizing signal components. The brightness or luminance signal component is a relatively wide-band signal and it is typically a combination of three primary color signals which are used in the color signal component.
In color television receivers, separate channels to the demodulator are provided for the brightness and the color signal components. The color or chrominance signals typically are transmitted with a band-width that is considerably less than the band-with of the brightness or luminance components. In addition, in most receivers the band-with of the color signal components often is further limited. For example, in a typical NTSC-type transmission system the brightness component (Y) is transmitted and received in its full band-width, and linear combinations of the color or chrominance signals are transmitted in a reduced band-width. This means that the fidelity of the color portion of the received and reproduced signal is inferior to the fidelity of the monochrome or brightness portion of the signal. As observed on the picture tube, this results in a smearing effect in the transitions from one color to another or in transitions between regions of different intensity within a field of the same color (for example, the variations of intensity of red in the various parts of a red rose.
The three different color signals in the United States are generally designated as color difference signals, that is color signals with the brightness component removed; and these signals are the red, blue and green color difference signals, designated respectively as (R-Y), (B-Y), and (G-Y). These three signals are transmitted in linear combination as chrominance signals designated as the I chrominance signal and the Q chrominance signal. The signal format is well known. It has been used for a number of years and the I and Q signals define separate perpendicular chrominance axes. The I and Q signals, as transmitted, have different band-widths (the I signal being of wider band-width) and each of these band-widths is substantially narrower than that of the Y or brightness signal components. In the interests of reducing the costs of television receivers as much as possible, most conventional receivers use compromises from the ideal characteristics and accept and reproduce somewhat erroneous chrominance or color signal components. In the processing of the chroma signals, the high frequency color components are lost due to the band-width restrictions of the receivers.
The public has long accepted color television reception which exhibits visible color infidelities. While these may be identified technically as being caused by incorrect highs, polarity reversals, and the like, they are visible on the display screen of the television receiver in the form of blurred or smeared transitions, desaturation of colors, and the like. These errors can be summarized as resulting in a picture that lacks crispness or detail. Many of these errors simply result from a slow response time of the color processing portion of the television receiver to transients from one color to another or from different intensities within a single color, particularly in sharp transitions of any type. Because of the limitations in band-width, these color transitions are slower than the transitions in the brightness or luminance portion of the signal; so that a "soft" or somewhat blurred picture is reproduced.
Various attempts have been made in the prior art to produce a color television display that has more detail than conventional displays. One such approach is disclosed in the patents to Richman, U.S. Pat. No. 4,181,917, issued Jan. 1, 1980, and U.S. Pat. No. 4,183,051, issued Jan. 8, 1980. The Richman systems add a high frequency signal component derived from the luminance or brightness signal component to the chrominance signal components. The effect is to attempt to precisely replace a missing high frequency component that was lost due to band-width limiting of the chroma signals. Another patent utilizing a similar technique is the patent to Faroudja, U.S. Pat No. 4,030,121, issued June 14, 1977.
Another system which has been developed to improve the crispness of chroma signals is disclosed in the patent to Davidse, U.S. Pat. No. 3,333,059, issued July 25, 1967. Davidse employs double differentiation of the chrominance or color signal to develop a correction signal which is added to the luminance signal components. Thus, the system of this patent is similar in its approach to the systems disclosed in the Richman and Faroudja patents.
Another system, which also uses the luminance signals to correct or improve the color signal components is disclosed in the patent to Netravali, U.S. Pat. No. 4,141,034, issued Feb. 20, 1979. The Netravali patent is directed to a system where the luminance or brightness signal transitions in a color video digital encoding system are used to predict the presence of similar transitions in the color component. This information then is used in the system to improve the color reproduction. Two other patents which operate on the color signal to alter the reproduced image are the patents to Nagaoka, U.S. Pat. No. 3,835,243, issued Sept. 10, 1974, and Keiper, U.S. Pat. No. 3,242,259, issued Mar. 22, 1966. Both of these patents detect the strength of the color signal and reduce the gain of the chroma channel under certain circumstances in an effort to improve the reproduced image.
The systems disclosed in these various patents, while capable of producing improvements in the details of the reproduced color television picture, require relatively complex additions to the television receiver. These additions result in increased manufacturing costs and introduce additional areas of factory and set-up adjustments to optimize the receiver performance. Accordingly, it is desirable to provide a system for color television receivers which is simpler than the systems mentioned above and which causes the reproduction of a picture on the screen of a color television receiver that appears to the viewer to have more detail than is presently available from conventional television receivers. In addition, it is desirable to accomplish this improvement of the reproduced image of a color television receiver without degrading the monochrome image or the reproduced color image quality in other respects.